This error is typically caused by a maximum concurrent connections limit on the SFTP server itself (either total concurrent connections or concurrent connections from a specific IP). To resolve the issue, this limit should be raised to a minimum of 25* connections.

* Cascade CMS uses a parallel publishing model where up to 2 publish jobs from every Site can be publishing at the same time. A publish job will use at most 4 connections and a minimum of 1 connection (depending on the number of concurrent jobs publishing at once).